1. Field of the Invention
This invention relates to a demodulating apparatus and a demodulating method, and more particularly, is applicable to a portable telephone.
2. Description of the Related Art
There exists a portable telephone as a mobile communication system which has spread remarkably. Various communication methods of the portable telephone have been proposed. The typical one is the code division multiple access (CDMA) method. The CDMA method has been proposed and put to practical use as the IS-95 (Interim Standard-95) Standards in the U.S. Recently, in the standardization project of the next generation mobile communication system called IMT2000 (International Mobile Telecommunication 2000) which is being carried out by the International Telecommunication Union (ITU), the CDMA method is also watched as a strong communication method in the next generation.
The CDMA method is a communication method using so-called spread spectrum communication method, in which at a transmitting side, a code sequence to be transmitted is multiplied by a spread code sequence having higher speed than that of the code sequence to be transmitted so as to spread the code sequence, and at a receiving side, the code sequence is inversely spread by using the same spread code sequence as the spread code sequence used at the transmission. The maximum period code sequence is generally used as the spread code sequence. The maximum period code sequence is a code sequence having the characteristics that a correlation between different codes is independent and the auto-correlation function is like impulse. Therefore, at the receiving side, the received code sequence can not be demodulated if it is not multiplied by the same spread code sequence as that of the transmitting side and having the same timing as that of the transmitting side. Thus, the CDMA method is a superior communication method in a confidentiality since the transmitted contents are difficult to be heard by a third party. Moreover, the CDMA method assigns a different spread code sequence for each mobile station, so that an interference problem will not occur even if the same frequency is used.
Hereinafter, the principle of the spread spectrum will be explained. "s.sub.i " denotes a transmission signal here. The transmission signal s.sub.i is a signal sequence of the complex number comprising a symbol length T.sub.sym. "c.sub.j " denotes a spread code. The spread code c.sub.j is a signal sequence of the complex number comprising a tip length T.sub.chip. "j" denotes a time series symbol number and the maximum value is "J". The spread code c.sub.j is the maximum period code sequence having the period J.sub.MAX, and the auto-correlation is like impulse. That is, the auto-correlation is obtained by the following equation (1): ##EQU1##
Note that "c*" denotes the conjugate of c and "%" denotes a residue arithmetic.
Further, the spread code c.sub.j is independent of the spread code c.sub.j ' having the same period J.sub.MAX as that of the spread code c.sub.j. This is indicated by the following equation (2): ##EQU2##
At the transmitting side, the spreading of the transmission signal si which is performed by using the spread code c.sub.j is indicated by the following equation (3): EQU x(iI.sub.sym +jT.sub.chip)=s.sub.i c.sub.i (3)
In this case, as shown in FIGS. 1A to 1C, because the symbol length T.sub.sym (FIG. 1A) of the transmission signal s.sub.i is extremely longer than the tip length T.sub.chip (FIG. 1B) of the spread code c.sub.j, the spread transmission signal x.sub.j is spread into a very wide area comparing to the original transmission signal s.sub.i (FIG. 1C).
On the contrary, at the receiving side, by using the spread code sequence which is the same code sequence as that of the transmitting side and has the same timing, the received code sequence is inversely spread by the following equation (4): ##EQU3##
and the transmission signal s.sub.i is demodulated. If the timing of the spread codes generated at the recording side deviates, the receiving side performs the inverse spreading by the following equation (5): ##EQU4##
and the transmission signal s.sub.i can not be demodulated. In this way, in the spread spectrum communication, the inversely spread codes which has the same code sequence as the code sequence used at the transmission side and has the same timing is convoluted into a reception signal, so as to demodulate th e reception signal.
In the portable telephone, the multipath fading where the reception level changes moment by moment occurs. Hereinafter, the multipath fading will be explained with referring to FIG. 2 and FIG. 3. An electric wave from a base station is reflected and diffracted by the buildings and then is transmitted as plural scattered waves. The portable telephone receives the plural scattered waves transmitted through respective transmission paths. For example, as shown in FIG. 2, the plural scattered waves 1 are received by an antenna 3 of an automobile 2 which has a portable telephone. The portable telephone then combines these plural scattered waves and demodulates them.
The transmission paths of the scattered waves respectively have the different transmission time so that the transmission characteristics respectively have a predetermined frequency response. This produces the linear distortion called inter-code interference in the reception signal. The inter-code interference is a phenomenon that occurs when at the receiving timing of a predetermined symbol, the influence of symbols before and after the symbol is added in accordance with the impulse response of the transmission path, so that a code decision error increases. Generally, the code decision error remarkably increases when the delay time .tau. of each scattered wave is the same degree and over as the tip length T.sub.chip of the spread code. Such transmission path of the scattered wave is indicated by the following equation (6): ##EQU5##
Note that "N" is the number of the scattered waves, "a.sub.n " is the complex gain showing the attenuated amount of each scattered wave and the phase rotation, and ".tau..sub.n " is the delay time of each scattered wave. These values of N, a.sub.n, and .tau..sub.n change at random. Thereby, the linear distortion also changes moment by moment.
As a receiving apparatus for compensating the linear strain caused by the multipath fading, there is a RAKE receiver. The RAKE receiver divides the multiwaves due to the multipath fading for each transmission path to generate plural scattered waves, and inversely spreads each of the scattered waves to combine them, thereby decreasing the linear distortion. Hereinafter, the RAKE receiver will be explained with referring to FIG. 3.
The RAKE receiver 10 inputs a reception signal S1 received by an antenna 11 to a high frequency amplifier 12. The high frequency amplifier 12 amplifies the reception signal S1 and outputs the resultant high frequency signal S2 to a frequency converter 13. The frequency converter 13 frequency-converts the high frequency signal S2 and outputs the resultant intermediate frequency signal S3 to an intermediate frequency amplifier 14. The intermediate frequency amplifier 14 amplifies the intermediate frequency signal S3 and outputs the intermediate frequency signal S4 to a filter 15. The filter 15 removes out-of-band unnecessary components or noise from the intermediate frequency signal S4 and outputs the resultant intermediate frequency signal S5 to an orthogonal detector 16.
The orthogonal detector 16 demodulates the intermediate frequency signal S5 based on the demodulation method of the orthogonal detecting method and outputs the resultant baseband signal S6 to a demodulator 17. The demodulator 17 inversely spreads the baseband signal S6 to generate demodulated data S7 and outputs this to an error correcting circuit 18. The error correcting circuit 18 error-corrects the demodulated data S7 and outputs the resultant demodulated data S8 to an audio decoder 19. The audio decoder 19 performs a predetermined demodulation processing on the demodulated data S8 and outputs the resultant audio signal S9 to an exterior through a speaker 20.
Here, the constitution of the demodulator 17 will be explained concretely with reference to FIG. 4. The demodulator 17 inputs the baseband signal S6 output from the orthogonal detector 18 to an analog-to-digital converter 30. The analog-to-digital converter 30 analog-to-digital converts the baseband signal S6 and outputs the resultant reception data S20 to a delay profile measuring circuit 31 and an inversely spread circuits 32A to 32C.
The delay profile measuring circuit 31 is generally called a matched filter and uses a finite impulse response (FIR) filter which is a digital filter. As shown in FIG. 5, the delay profile measuring circuit 31 is composed of a delay circuit 31A comprising a plurality of delay devices, a multiplying circuit 31B comprising a plurality of multipliers corresponding to the delay devices, and a combining circuit 31C for combining the multiplied results output from the multiplying circuit 31B. Reception data S20 comprising a bit sequence is input to the delay circuit 31A. the delay circuit 31A successively shifts the bits of the reception data S20 and outputs them to the multiplying circuit 31B.
The multiplying circuit 31B multiplies the bits output from the delay circuit 31A by the codes respectively set in the multipliers constituting the multiplying circuit 31B, and outputs the multiplied results to the combining circuit 31C. The combining circuit 31C combines the multiplied results output from the multipliers of the multiplying circuit 31B to obtain the correlation value. In this way, the delay profile measuring circuit 31 inversely spreads each scattered wave included in the reception data S20 one by one and measures the power level, so as to generate the delay profile S21 showing the power level distribution of each scattered wave to the delay time, to output this to an assigning circuit 32.
The assigning circuit 32 selects the scattered wave one by one from the scattered wave having the highest power level among a plurality of scattered waves based on the measured delay profile S21, and outputs timing signals S22A to S22C showing the reception timings of the selected scattered waves to corresponding inversely spread circuits 32A to 32C.
For example, as shown in FIG. 6, the delay profile measuring circuit 31 produces the delay profile S21 showing the distribution of the power level of scattered waves to the delay times .tau..sub.1 to .tau..sub.7. Since the delay profile S21 is determined by topography, streets, and so on, it is constant between several tens milliseconds and several seconds. The assigning circuit 32 selects the scattered waves of the delay times .tau..sub.3, .tau..sub.4, .tau..sub.5 among the scattered waves of the delay times .tau..sub.1 to .tau..sub.7 based on the delay profile S21 to generate the timing signals S22A to S22C representing the reception timings.
The inversely spread circuit 32A generates a pseudo-noise code of a timing based on the timing signal S22A and inversely spreads the reception data S20 by using the pseudo-noise code. Thereby, the inversely spread circuit 32A inversely spreads only the scattered wave based on the instruction of the assigning circuit 32 among a plurality of scattered waves, and outputs the resultant inversely spread data S23A to the combining circuit 33.
Similarly, the inversely spread circuits 32B and 32C respectively generate pseudo-noise codes having a timing based on the timing signals S22B and S22C and inversely spread the reception data S20 by using the pseudo-noise codes generated. Thereby, the inversely spread circuits 32B and 32C inversely spread only the sca ttered wave based on the instruction of the assigning circuit 32 among a plurality of scattered waves, and output the resultant inversely spread data S23B and S23C to the combining circuit 33.
The combining circuit 33 synchronizes the timings of the inversely spread data S23A to S23C, and then combines the inversely spread data S23A to S23C by the maximum ratio combining method to output the resultant combined data S24 to a code demodulating circuit 34. The code demodulating circuit 34 demodulates the combined data S24 based on a predetermined demodulating method, and outputs the resultant demodulated data S7 to the error correcting circuit 18 (FIG. 3) at a later stage.
As described above, the demodulator 17 divides the baseband signal S6 into scattered waves different from each other in the delay time to inversely spread them, and then combines them again with the delay time and phase being matched. This suppresses the generation of the linear distortion. More specifically, the reception signal y(t) is obtained from the above described equation (6) by the following equation (7): ##EQU6##
Note that "*" denotes the convolution arithmetic. However, the reception signal y(t) is not inversely spread if the inversely spread sequence and the timing are not matched. Then, the demodulating circuit 17 respectively inversely spreads the scattered waves different from each other in the delay time, compensates the phase of the complex gain a.sub.n, and matches the timing of the inversely spread output so as to combine the reception signal y(t). As shown in the following equation (8): ##EQU7##
the combined signal y'(t) suppresses the linear distortion. In this connection, in the demodulating circuit 17, since the number of the inversely spread circuits 32A to 32C which can be provided is limited, the inversely spreading can not be performed on all of the scattered waves.
In the delay profile measuring circuit 31 described above, the period of the measuring time is generally determined by the observation value of the delay profile based on the communication system or topography. Normally, if it is set to several tens microseconds, the delay profile may be measured widely from a city area through a mountainous area. Therefore, the delay profile measuring circuit 31 widely sets the measuring time so as to measure the delay profile in any situation.
However, the period of time where the delay profile distributes is determined by topography or buildings. For instance, it is known that the delay profile in a city area has a relatively small extent. If the measuring time of the delay profile is set widely and fixed as the delay profile measuring circuit 31, the measurement is performed in the period of time wider than it is needed regardless of the delay time of the scattered wave. Accordingly, the measuring time becomes long and the power consumption of the delay profile measuring circuit 31 becomes large.
To use the portable telephone operated by a battery for a long time, it is necessary to reduce the power consumption of the portable telephone. Since the delay profile measuring circuit 31 is driven even in a waiting time, if the power consumption of the delay profile measuring circuit 31 can be reduced, the power consumption of the entire portable telephone can be reduced. In the delay profile measuring circuit 31, there arises a problem that the power consumption increases when the measuring time becomes longer than it is needed, since the power consumption is desired to be reduced.